Improving Performance Characteristics of Poly (Lactic Acid) (PLA) Based Nanocomposites by Enhanced Dispersion of Modified Cellulose Nanocrystals (CNCs)
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Poly(lactic acid), (PLA) is a biodegradable and biocompatible polymer which has attracted significant attention as a promising substitute for petroleum-based polymers. To optimize the usage of PLA in a wide range of applications, different methods such as polymer blending and the incorporation of traditional and nanofillers have been extensively explored. Cellulose nanocrystals (CNCs), rod-like nanoparticles with a perfect crystalline structure, are considered as outstanding reinforcing agent owing to the excellent mechanical properties. The optimal characteristics of CNCs as a reinforcing agent in the polymer can be achieved through homogeneous dispersion within the polymeric matrix. However, the strong hydrophilic character of CNCs due to the presence of hydroxyl groups on the surface restricts the uniform dispersion of CNCs in the PLA matrix. In this work, three surface modification treatments along with two different mechanical preparation techniques were employed to improve the dispersion quality of CNCs in the PLA matrix. Polymer adsorption, green esterification, and time-efficient esterification were used as surface modification treatments. Solvent casting and spin-coating method were employed to prepare highly concentrated CNCs masterbatches. Nanocomposites were prepared using melt extrusion, followed by an injection molding process. The morphology of masterbatches indicated better CNCs dispersion through spin-coated thin films, suggesting a high evaporation rate and the effect of centrifugal force and surface tension in the spin-coating process decrease the possibility of CNCs aggregate through the film. Consequently, nanocomposites manufactured using spin-coated masterbatches exhibited higher mechanical strength in comparison with solvent cast ones. In the case of surface modification treatments, the most uniform CNCs dispersion was observed in the nanocomposites reinforced by valeric acid through esterification technique. Higher thermal stability was also achieved through the application of esterification technique. This observation was related to the presence of DMAP on the surface of CNCs which turns into inert materials, prohibiting the thermal degradation. The higher molecular weight and lower molecular number observed in spin-coated samples in comparison with film cast nanocomposites led to the higher damping behavior in spin-coated nanocomposites. This observation indicated the more viscoelastic properties in spin-coated samples owing to the presence of more polymer chain freedom in spin-coated nanocomposites.